And a few screenshots to illustrate (Click the images to view real size):
Before shader effect (zoomed in at shadow border):
After shader effect (zoomed in at shadow border):

Over-all view before:
Over-all view after:

You still see a bit of aliasing, thats because I needed to do a Gaussian blur and I did a box blur. Online they say when running the box blur three-times it produces a blur that looks strongly like Gaussian Blur. But I’m happy with the results now.